The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPPthird generation partnership projectBSICbase station identification codeDLdownlinkE-UTRANevolved UTRAN (LTE)FACHforward access channelGERANGSM-enhanced data rates for global evolution (EDGE)GSMglobal system for mobile communicationsHSPAhigh speed packet accessLTElong term evolutionRATradio access technologyTDDtime division duplexUEuser equipmentULuplinkUTRANuniversal terrestrial radio access networkWCDMAwideband code division multiple access
The exemplary embodiments detailed herein are in the context of the WCDMA and HSPA (GSM) wireless systems to resolve problems in measuring inter-frequency and inter-RAT neighbor cells. These teachings are not limited only to those wireless systems but are more generally applicable; the examples merely illustrate specific implementation details relevant to those systems.
In the WCDMA/HSPA system the UE can make these inter-frequency and inter-RAT neighbor cell measurements when in the FACH state, which is when the UE is camped on a cell and has a signalling connection established with the network. The UE makes such measurements only during what is termed a measurement occasion. Currently, the inter-RAT measurement occasions are specified for GERAN only, but as E-UTRAN becomes more ubiquitous these neighbor cells are expected to be measured by the UE camped in the WCDMA/HSPA system also. A problem arises when increasing the number of measurement occasions to include E-UTRAN.
Generally, measurement occasions are infrequent and are shared equally between all the measurement types configured for the UE, so the effectiveness of these measurement gaps is quite poor. When E-UTRAN is introduced for the inter-RAT measurements the effectiveness may become even worse if the current measurement occasion concepts are simply extended to include E-UTRAN neighbors.
First, consider the current measurement occasion practice which is set forth at 3GPP TS 25.133. The measurement repetition Tmeas in milliseconds (ms) is determined by the following algorithm:Tmeas=[(NFDD+NTDD+NGSM)·NTTI·M—REP·10];                where:        M_REP is the measurement occasion cycle length where K is given in Table 8.10A of 3GPP TS 25.133 (K is the FACH measurement occasion length coefficient, which is specified in 3GPP TS 25.331)        
The FACH measurement occasion of NTTI frames will be repeated every NTTI*M_REP frame. This means that the measurement time Tmeas increases uniformly for each RAT supported, which has a detrimental impact on inter-frequency and inter-RAT measurements and therefore UE mobility. Since currently only GERAN neighbor cells account for the inter-RAT measurements this has not yet become a problem in practice.
To quantify the impact of adding E-UTRAN cells to the inter-RAT measurements, consider a typical FACH configuration as follows: inter-frequency (NFDD=1); inter-RAT (GERAN) (NGSM=1), where K is 3 (MREP=8) and NTTI=1. In this configuration, Tmeas=(1+0+1)*1*8*10=160 ms.
In this scenario there is an inter-frequency measurement occasion every 160 ms, but since it takes about five measurement occasions to perform a search then there can be a search only every 800 ms. Also in this scenario a GERAN (inter-RAT) measurement occasion is also configured every 160 ms, which as seen at FIG. 1A yields a BSIC verification time of 7.68 seconds and at FIG. 1B a BSIC refresh time of 6.4 seconds.
Now extend this same measurement occasion protocol to include the possibility of E-UTRAN neighbor cells. In this straightforward extension the measurement time Tmeas in milliseconds is then defined as:Tmeas=[(NFDD+NTDD+NGSM+NEUTRA)·NTTI·M—REP·10]Using the same FACH configuration as above then Tmeas=(1+0+1+1)*1*8*10=240 ms.
There is therefore an inter-RAT measurement occasion for E-UTRAN every 240 ms, but in this case it takes as few as one measurement occasion to perform an E-UTRAN search so there is a search every 240 ms. This also provides an inter-frequency measurement occasion every 240 ms, and since it still will take about five measurement occasions to perform a search then there can be an inter-frequency search only every 1200 ms.
The inter-frequency measurements would be impacted by including E-UTRAN because the number of cell-FACH measurement occasions is reduced by a third. This also results in a GERAN measurement every 240 ms, which results in a BSIC verification time of 29.76 seconds as seen at FIG. 2A, and a BSIC refresh time of 17.28 seconds as seen at FIG. 2B. This is seen to be too long of a time for GERAN measurements. The teachings below address this problem, but as indicated have utility beyond only the GSM/GERAN/E-UTRAN systems which are used only for specific illustration of the principles.